mmc: dw_mmc: Avoid hung state if GEN_CMD transfer fails

If we get a data error during a block transfer command, a stop command
(CMD12) is normally initiated. But this does not work for the general
command (CMD56), but instead the action is ignored and an uninitialized
command struct is used for the stop action, with unexpected result.

Fix this by adding a check for GEN_CMD when preparing stop transmission.

Signed-off-by: Mårten Lindahl <marten.lindahl@axis.com>
Link: https://lore.kernel.org/r/20211103182716.28419-1-marten.lindahl@axis.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>